Power saws with two reciprocating blades moving in opposite directions in a groove in a guidebar powered by some driving means as electrical or gasoline engines are known long ago from among others the patents Northall U.S. Pat. No. 1,491,134 and Kirksey U.S. Pat. No. 2,840,136.
It is known from such designs to let the blades touch each other to make a narrow cut, and also to make at least the majority of teeth such that their setting does not let them cross or intersect the plane of contact between the two blades.
Various tooth shapes have been used, including symmetrical crosscutting teeth with beveled side edges and a sharp point as in U.S. Pat. No. 1,491,134 and drag or raker teeth with a main cutting edge at the tip as in Merkur U.S. Pat. No. 2,659,969. Various combinations of these are also known, as in U.S. Pat. No. 2,840,125 and Swedish Pat. No. 155,367.
The saws mentioned have mostly been intended for soft materials with fibrous structure, such as wood, meat or bread, where fragments of the sawn material getting caught between a pair of teeth moving in opposite directions are easily deformed or sheared without hindering the motion of the sawblades.
For sawing of material containing hard grains or fibers, such as porous concrete and fiber reinforced plastics, sawblades with these types of teeth are unsuitable, since they will only tend to loosen the grains or fibers from the sawn material, but are not efficient in shearing large grains or fibers which are still partly attached.
For the special case of cutting plaster casts used in surgery and incorporating layers of cotton gauze, it has been suggested in U.S. Pat. No. 2,659,969 to use taper ground blades provided with unset teeth with main cutting edges for the plaster and side cutting edges abutting the other blade to shear the cotton fibers between pairs of teeth moving in opposite directions.
If the grains or fibers are harder than in a plaster cast, each such shearing requires a large driving force, and if several grains or fibers are sheared simultaneously, the total force might stall or damage the driving means. This invention avoids overloading of the driving means by shaping and locating the teeth of the sawblades in such a manner that only one pair of teeth at a time is in a shearing position requiring a large driving force.